Question on creating a spiral
That's true.. the Screw modifier doesn't seem to know how to display itself in Edit Mode, unless there are faces to display. 

The screw of a single Mesh vertex is there, if viewed in Object Mode.. and if the modifier is applied, a valid spiral of edges results. (Not so for a single Curve point, where the modifier can't be applied in any case)

If you need to be able to see a screwed single vertex in Edit Mode, a get-around is to make a small E extrusion from it while editing, and delete it when no longer needed.

This does seem anomalous behavior.. but then, so many settings in the modifier deal with faces.. perhaps this case was overlooked, or considered too awkward?